1. Field of the Invention
This invention relates to a method for controlling the pressure ratio of a jet pump for the purpose of regulating a predetermined operating or working vacuum.
2. Discussion of Related Art
In a jet pump, for example a steam jet pump, a gas jet pump or a water jet pump, the flow energy of a motive fluid through nozzles and diffusors is used to aspirate or compress a fluid to be delivered. Gases, vapors and liquids are used as the motive and delivery fluids. Jet pumps are easy to make and have no moving parts, but are relatively inefficient and become even more inefficient in the event of changes in the operating conditions, for example the pressures and delivery volumes. In view of the difficulties involved in the generally intermeshed or multiloop regulation of a jet pump, jet pumps are operated at constant motive fluid pressure in practice, the surplus energy being destroyed by throttling, addition of a foreign gas, or by acceptance of a lower reduced pressure or suction pressure than that required.
Jet pumps, preferably steam jet pumps, are used for example as suction pressure generators in vacuum distillation. With plants as complicated as these, a prescribed behavior profile is intended to be achieved simultaneously for a number of controlled variables. However, these variables are interdependent. In addition, every regulating intervention influences the other controlled variables, in general to a more or less considerable extent. Accordingly, it is only possible to use intermeshed rather than separate controllers. The problems of autonomy, invariance, controllability and observability involved in multiple control systems of this type makes the use of conventional P, I, PI and PID control techniques and the like hypothetical at least on economic grounds.
In a jet pump of given dimensions, the ratio of motive fluid volume G1 to delivery fluid volume G2 is a function of
Pe=motive fluid pressure, PA1 Pa=pressure at exit of jet pump, and PA1 Po=suction pressure.
Accordingly, for a predetermined suction pressure Po and a predetermined volume of delivery fluid G2, the necessary volume of motive fluid G1 is only dependent on a function of Pe and Pa. Since this function has the form of a pressure ratio, the expression "controlling the pressure ratio" in the context of the method according to the invention means the control of Pe and/or Pa.